Unlike outdoor azaleas and rhododendrons, indoor azaleas are not frost hardy and are therefore not suitable, in frost-prone areas, for permanently planting in the garden when their period of flower is over. However, they can be kept and grown on as pot plants for flowering in future years.
Growing indoor azaleas as houseplants
Keep flowering plants in a cool, humid atmosphere with plenty of light, such as an unheated porch. Plants will deteriorate if they suffer long periods in hot, dry conditions.
Remove the dead flowers. Repotting can then be done if necessary, increasing the pot size by no more than 2.5-5cm (1-2in) in diameter. Most indoor azaleas are imported from the Continent. They are usually growing in a pine needle, peat and leafmould medium, but a suitable alternative medium can be made from equal parts leafmould, peat substitute such as composted bark, and lime-free sharp sand. Acidic, partially decayed pine needles can be included if available; the essential point is that no alkaline materials should be used. A proprietary, ericaceous or lime-free compost is also suitable. Water-in using rainwater.
Where possible keep the plant in a warm greenhouse or conservatory, with a temperature of 13-16°C (55-61°F). Mist daily and water as needed to encourage new root and shoot growth. Alternatively, give the protection of a frost-proof cold frame or greenhouse, where growth initially may be slower.
For plants kept indoors, continue to house the plant in a cool, humid room until after the risk of frosts has passed in your area.
Harden off plants in May by transferring them from warm to cooler conditions and standing them outdoors when danger of frost is past. Plunge the pots to the rim in a bed of peat or peat substitute to avoid the danger of frequent drying out.
If plunging in soil, place some coarse, gritty material at the base to discourage worms from entering the pot through drainage holes. A cool, partially shaded position is desirable, but in deep shade flower buds may not form.
Flat dwellers should try to find a cool, moderately shady position in the home, or on a balcony, perhaps shaded by other plants such as climbers.
Bring under cover in September before any risk of frost, and place in a position of good light, avoiding direct, scorching midday sun. Ideally site in a cool, moist area of the greenhouse with night temperatures in the region of 7-10ºC (45-50ºF).
Watering and feeding
While plants are growing indoors, never allow the compost to dry out completely. If it does, soak in a bucket of water until no more air bubbles rise from the compost. Do not leave soaking for an excessive period of time as this can drown the roots.
Careful watering with rainwater is essential throughout the summer; if plants are left to suffer drought conditions, growth may be stunted and flower buds may fail to develop or may only partially develop, eventually falling unopened or shrivelling on the plant. Avoid using tap-water particularly if you live in a hard-water area. This contains dissolved sodium bicarbonate which is alkaline and is toxic to the plant.
Azaleas do not generally require extensive feeding, but when the first signs of new growth are seen, a general purpose proprietary liquid fertiliser may provide a useful stimulus. Apply at half the recommended rate for pot plants on two or three occasions up until the end of June. It is not necessary to feed in the first growing season after repotting.
There will be less demand for water as the weather gets cooler, but water requirements will increase again as buds swell and flowers open. Ventilate freely with rising day temperatures. Commercially, plants are brought into flower early by forcing in warm, humid conditions, with temperatures approximately 18-21ºC (65-70ºF) for six to eight weeks. Therefore, plants grown by the amateur gardener will be later to bloom.
A point to remember is that they will need to be grown on in a greenhouse if they are to bloom as profusely as when initially purchased. In addition, following forcing, it may be a year or two before they flower well, if a greenhouse or conservatory is not available.